The requirements imposed on communication infrastructure installations, such as private branch exchanges for example, are constantly increasing. The cause for the increasing requirements with regard to the data transmission capacity of switching devices lies in the constantly increasing demand for voice, video and data communication and the consequence that networks of broader bands have to be used for establishing connections. The cause for greater flexibility with regard to the number of subscribers which can be connected to switching devices lies in the requirement for the infrastructure to keep pace technically with the flexibility of the business processes of the users of the communication device. This results in a great demand for flexible modularly expandable private branch exchanges.
Current devices are based on time-slot multiplexing connections between communication terminals which are set up by means of a switching unit, for which purpose commands which indicate which defined time slot of an incoming connection is assigned to which defined time slot of an outgoing connection are generated by a control device. Such switching units are generally suitable for the establishment of a defined number of connections. The number of these connections is in this case dependent on the current demand of a private branch exchange. It is generally in the range of at most several thousand incoming and outgoing connections. Consequently, such devices are not particularly well suited for flexible adaptation to growing numbers of subscribers. Similarly, the data transmission capacity per time slot of a connection is restricted by the ISDN standard (Integrated Services Digital Network) to a maximum of 64 KB. This specified limit hinders, or prevents, a flexible subscriber-specific adaptation of different data rates for each connection.
Moreover, in the case of current devices, the setting-up of a communication infrastructure in the form of a network of decentralized devices which are supplied with messages by a central control is restricted because strict time requirements have to be satisfied when transporting control messages and, as from a defined length of the control line, it is no longer possible to comply with them. Used at present on these message lines is the HDLC protocol (Highlevel Data Link Control), with which messages are transmitted with the function, for example, of controlling the access of individual units in the decentralized devices to a PCM data stream (Puls Code Modulation), in that they prescribe defined time slots. If HDLC connections were simply lengthened, the time requirements between the communication partners involved at the end of the link cannot be satisfied. The communication partners would have to be modified in such a way that they impose lower requirements on the time response. This is not practicable, since many possible communication partners are concerned and consequently great expenditure is incurred and the communication partners would have to be provided with more resources, for example memories.
FIG. 1 shows an example of a known private branch exchange 150 with a central control device. This private branch exchange is connected to two peripheral devices P1 and P2, to which there is respectively connected a communication terminal KE1 and KE2 operating on a digital or analog basis. These peripheral devices P1 and P2 are accommodated in the same spatial area as the first central device Z1. They are consequently located in the same space or in the same cabinet as it. The terminals occupy defined time slots of a PCM data stream (Puls Code Modulation) with communication data. In this case, these analog or digital communication terminals KE1 and KE2 are connected via interface modules SLMO1 and SLMO2, which feed to the PCM data stream, or remove from it, data which are intended for the respective terminals, or come from the respective terminals, via time slots established by control messages. Two PCM data streams are denoted in the figure by 100 and 200, respectively. Likewise represented are signaling connections 110 and 210, via which message traffic with a central control can take place. In the case of this representation it should be noted that a logical representation of the connections is shown in the topology for individual connections, and that this is not a physical representation. In the technical realization of these networks, the transport data and the messages can be transmitted over the same connection medium without restriction.
Also represented are peripheral devices P1 and P2, and also the supply modules LTUC1 and LTUC2, which regulate the data traffic to the interface modules, for example SLMO1 and SLMO2, of the respective peripheral devices. In this case, the peripheral device is fed control messages via the line 110 and the peripheral device P2 is fed control messages via the line 210. It can be clearly seen in the case of this known private branch exchange that, with this arrangement of the individual components of the switching device, both the information to be transported and the signaling information, exchanged by means of coordinated message traffic, have to be fed to a central device ZE1.
To be specific, messages 2, which are to be exchanged between the central device ZE2 and the peripheral devices P1, P2, are collected and distributed by a message device DCL. The setting-up and clearing-down of connections is controlled by means of the Call Processing CP, with the Call Processing using for this purpose, for example, device-specific interface functions DH, which are realized for example in the form of program modules. In particular, setting commands 1 for the switching unit MTS are generated. Such a setting command essentially controls which input of the switching unit is to be connected to which output in order to provide a communication connection via this switching device. In such a known communication arrangement, control and connection functions are consequently performed by a single spatially integrated functional unit of the communication network. In the case of such a center-oriented configuration, problems arise because the data to be transported have to be fed to the central device ZE1. This is the case even if, for example, two communication terminals which are connected to the same peripheral device P1 want to communicate with each other. Such a centrally oriented arrangement also gives rise to high expenditure on cabling, because both the control lines and the communication lines have to be routed to the central device ZE1. It is not possible for peripheral devices to be distributed over a wide area, because the time-critical message traffic via the control lines with the aid of a HDLC protocol cannot take place over links comprising lines of any desired length. To be able to achieve a greater area coverage by means of such devices, the coupling of a number of devices would be conceivable, although the advantages of a single system in the form of central interfaces, and for example central facility control, would be lost. Furthermore, when linking them up, additional trunk modules would have to be installed and additional connecting cables would have to be laid for their connection. Such private branch exchanges also cannot be modularly expanded to whatever extent is desired, because the switching unit MTS for example can only be provided as a complete unit. This means that, in an extreme case, a new switching unit with, for example, 4096 ports must be purchased and installed for a single additional connection. The transmission rate in such systems is limited for example by the possibility that only a maximum of 64 kbits, or some other administratively fixed or technically dictated volume of data which is prescribed by the ISDN standard, can be transmitted per time slot. In this case, different data rates for individual communication connections are not possible.